How to Kill Caterpillars on Oleanders

Judy Wolfe

Shrubs as tough, colorful and deadly as James Bond are rare, but evergreen oleanders (Nerium oleander) make the list. The Mediterranean and Asian natives have adapted to conditions in some parts of U.S. Department of Agriculture plant hardiness zones 8 through 10 well enough to crowd out native plants. They also contain sap so deadly that eating just one leaf may be lethal. Oleanders' superhero strengths, however, can't deter their equally tough and colorful foes: Oleander caterpillars (Syntomeida epilais Walker) dine on the shrubs at will. Killing them requires persistence and timing.

The Gaudy Gluttons

In one month, oleander caterpillars eat their way from 1/10 to 1 1/4 inches in length, a nearly 13-fold increase. As newly hatched caterpillars, they feed in colonies on the backs of the leaves.

After their third molt, the colonies disperse and each caterpillar moves over the plant, eating entire leaves as it goes. At this stage, their black-bristled, bright-orange bodies are clearly visible. The pests molt three more times over the next three weeks, and heavy infestations may defoliate an oleander.

Meet the Moths

The insects responsible for oleander caterpillar infestations are polka-dot wasp moths. The iridescent, blue-black moths lay clusters of creamy-white to light-yellow eggs on the backs of their host plant's leaves in spring.

By flying during the day, polka-dot wasp moths break with normal moth behavior. They're now found wherever oleanders grow outside of California. While the shrubs remain their most common caterpillar hosts, the moths also lay eggs on other toxic dogbanes.

Early Elimination

Killing oleander caterpillars is easiest when they're still feeding in groups. On small oleanders, snip off the infested leaves with clean, sharp pruning shears and drop them in a sealable plastic bag. After freezing the bag for 24 hours, put it in the trash.

Protect yourself from the skin-irritating sap with waterproof gloves, long pants and a long-sleeved shirt. To avoid spreading disease, wipe the pruner blades down with a cloth dipped in rubbing alcohol between cuts.

Kill mature caterpillars by handpicking and dropping them in soapy water.

Battle Back With Bugs

Tiny predatory wasps and tachinid flies both lay their eggs on oleander caterpillars, and their larvae devour the pests from the inside out. To lure them to your oleander bushes, plant parsley (Apiaceae) family plants nearby.

The adult flies and wasps arrive to feed on the plants' nectar or pollen and stay to deposit their eggs. Suitable perennials include yarrow (Achillea millefolium) and fennel (Foeniculum vulgare), hardy in USDA zones 3 through 9 and 4 through 9, respectively.

Microbial Magic

Ready-to-use Bacillus thuringiensis var. kurstaki spray kills early-stage oleander caterpillars with blood-poisoning microbes. Monitor the plants daily for eggs on the backs of the leaves, and spray when the last of them has hatched.

Use the Bt when no sun is hitting the plants. Spray until it drips from both sides of the leaves, and repeat after rain or every five to seven days as long as you see active caterpillars.

Dress in protective clothing, safety goggles and a respiratory mask, and follow the label's instruction when using the Bt.